1. Field of the Invention
The present invention relates generally to processes for producing and recovering organic acids, such as lactic acid, and/or organic acid amides, such as lactamide. More particularly, it concerns methods that rely on azeotropic distillation for production and recovery of organic acids and/or organic acid amides.
2. Description of Related Art
Organic acids such as lactic acid have a number of commercial uses, for example in food manufacturing, pharmaceuticals, plastics, textiles, and as a starting material in various chemical processes. The current market in the United States for one organic acid, namely lactic acid, is about 50,000 tons per year, more than half of which is imported. It is well known to produce organic acids by fermentation of sugars, starch, or cheese whey, using microorganisms such as Lactobacillus delbrueckii to convert monosaccharides such as glucose, fructose, or galactose, or disaccharides such as sucrose, maltose, or lactose, into organic acids such as lactic acid. The broth that results from fermentation contains unfermented sugars, carbohydrates, amino acids, proteins, and salts, as well as the acid. Some of these materials cause an undesirable color or can interfere with downstream processing of the organic acid. The acid usually therefore must be recovered from the fermentation broth and in some cases must undergo further purification before it can be used.
Commercial uses of organic acid amides such as acetamide, formamide, and lactamide are typically as starting materials for various chemical processes. For example, formamide is used commercially as a solvent for certain materials due to its strong hydrogen bonding capability. Organic acid amides can be formed by heating an ester of an organic acid in the presence of ammonia. Organic acid amides can also be formed by heating the ammonium salt of an organic acid. For example, acetamide can be produced in this way. Formamide can be produced in a variety of ways. One method that can be used to produce formamide involves heating lactamide in the presence of formic acid.
Lactic acid and other α-hydroxyacids exist in two different optical isomers. For the example of lactic acid, these isomers are L-(+)-lactic acid and D-(−)-lactic acid. An equal mixture of D and L lactic acids is called a racemic mixture. It is often desirable to produce lactic acid with a high proportion of only one of the optical isomers. Different microorganisms used in fermentations to produce organic acids can produce different proportions of optical isomers of a particular organic acid. Chemical synthesis to prepare a higher proportion of a particular optical isomer can be difficult. It is desirable to minimize reactions that lead to the conversion of L-(+)-lactic acid into D-(−)-lactic acid and vice versa, so called racemization reactions. (L-(+)-lactic acid is also designated as S-(+)-lactic acid. D-(−)-lactic acid is also designated as R-(−)-lactic acid.) Exposing lactic acid solutions to relatively high temperatures can increase certain racemization reactions.
Lactamide and certain other substituted amides can exist in two different optical isomeric forms. For the example of lactamide, which is the amide typically formed from ammonia and lactic acid, these isomers are S-(−)-lactamide and R-(+)-lactamide. An equal mixture of R and S lactamides is called a racemic mixture. Note that the (+) and (−) designation refer to the optical rotation of a beam of polarized light that is passed through a standardized solution of the chemical, while the R and S notation refer to the stereo-specific configuration of the molecule. Thus, when S-(+)-lactic acid is converted to lactamide, the resultant lactamide is S-(−)-lactamide.
During the production of an organic acid such as lactic acid by fermentation, the increasing concentration of the acid in the fermentation broth reduces the pH. As the pH decreases, the growth of the microorganism is inhibited and eventually stops, and therefore acid production stops. To prevent this, the pH of the fermentation broth typically is controlled by adding a base for neutralization, such as ammonia or a sodium or calcium base. However, one result of the addition of such a base is the formation of a salt of the acid (e.g., ammonium lactate). Therefore, it is often necessary to convert the salt to free acid or another form such as an ester, which subsequently can be converted to the free acid.
Lactic acid is one organic acid of particular interest today because of a great projected demand for use as a polymer feedstock, particularly for use in producing degradable plastics. It is also used in the pharmaceutical and food industries, in leather tanning and textile dyeing, and in making solvents, inks, and lacquers. Although lactic acid can be prepared by chemical synthesis, production of lactic acid by fermentation of starch, cane sugar, whey or certain other carbon sources is a less expensive method. The production of lactic acid by fermentation is most efficient at a pH range where the lactic acid is largely present as a salt. Thus recovery of pure lactic acid often requires conversion of the salt into free acid and additional purification steps. One method that is used in purification is the production of a lactate ester from the lactic acid or salt, followed by purification of the ester. Finally the ester is converted to the free acid.
Lactic acid or other hydroxyacids or diacids can be converted to polyesters. These polyesters can be recycled via digestion using pressurized water, acid, base, or a combination of such treatments. The products of such digestion can be a mixture of organic acids, salts of organic acids, amides of organic acids. This digested recycled material can contain significant impurities and require purification to recover the organic acids or amides therein.
Additionally, during processing of ammonium salts of lactic acid, there is a tendency for lactamide to form via the following reactionLactic acid+ammonia→lactamide+water
Such organic acid amides can be formed in fermentation broths. Amides are typically purified by distillation or crystallization. Hydroxyamides, such as lactamide, can have relatively high boiling points and can be difficult to distill. Thus, there is a need for a better process to purify amides, in particular, hydroxyamides and other substituted amides.
There is a long standing need for improved processes for the production and recovery of relatively pure organic acids and organic acid amides, particularly lactic acid and lactamide.